There are birds, like this Great Blue Heron, that have several separated feathers at the wingtips. This is a bit like having propellers pushing the air down at the wingtips. It helps prevent higher pressure air below the wing from sneaking around the end of the wingtip to the low pressure area above the wing. Click pictures to enlarge them.
Thanks to David Stokely for the two pictures above.
A few other pictures showing this type of wingtip.
This video shows some large birds with this type of wingtip.